[DeTomaso] drilled bolts for brake calipers

[JDeRyke at aol.com]

In a message dated 8/6/09 11:02:02 AM, kenn_green at yahoo.com writes:
snip....
> is 1/16 (0.0625") OK for safety wire?  Seems like cobalt drills would be 
> a good choice.  But I thought that cap head screws were usually grade 8.
> 
As long as the bolt head is big enough to safely accept it. The 
aircraft-grade safety wire of choice is 0.032" dia, so drilling an 0.060" hole is far 
bigger than necessary, but in these bolts it should work OK. An 0.060" drill 
is maybe slightly less prone to break than an 0.048", I guess. Stock caliper 
bolts are metric gr 8.9, roughly equivalent to our SAE gr-5, so no problem 
drilling them. 
 
High-carbon bolts (gr 5 or 8) drill fine as long as -A- the drill is sharp, 
-B- you use lots of lube and -C-use the right drill speed. Too fast is 
worse than too slow 'cause it builds heat. Once the drill bit gets hot or dull- 
which happens pretty quickly, the bottom of the hole will overheat and that 
hardens the steel in & around the partial hole. Then, you've really got a 
problem finishing up that hole, even with a new bit and the hole full of oil. 
Sometimes, its better to start over on a different flat with a new drill 
bit. As I said, have a pocketful of new bits close by when you start such a 
project drilling many bolts and don't try stretching a worn bit for 'one more 
hole', which will only end in tears... 

You guys know not to drill straight thru a hex-bolt head.... the technique 
is, you start in the middle of a flat, drill in a very short distance, 
rotate the bolt head so the drill is now angled to the surface and drill in a 
little more, using the initial hole to catch the bit so it doesn't slide off. 
Then rotate the bolt still more & continue.... if done right, the hole will 
go in one flat and out the next one, not clear across the head. All this is 
slow & tedious and is why people sell drill guides, but it can be done 
satisfactorily either way. 

With allen bolts, you drill one hole straight through one side into the 
center hex hole, again not all the way across and try to not hit a corner of 
the internal hex. If you do, you may have to pound a hex-driver in the 
hex-hole (before adding the wire) to deburr it enough to tighten it. Normally, 
they're much simpler than drilling hex bolts, even though black-oxide allens are 
almost always gr-8. But there are so many chinese-copies floating 
around..... Stainless ones can be almost any grade and are not often marked, so out 
of the box, you never know for sure.   Good luck- J Deryke